This invention relates to a process and apparatus for treating workpieces with a treating medium in the vapour phase. The invention, in one embodiment, is more specifically directed toward a process and apparatus for treating workpieces in a subatmospheric environment with a treating medium in the vapour phase. Preferably, the treating medium is in an unsaturated vapour phase. In another embodiment, the invention is more specifically directed toward a process and apparatus for treating workpieces with a treating medium in an unsaturated vapour phase.
The invention is particularly useful in the soldering of components to printed circuit boards and more specifically to reflowing pre-deposited solder cream to solder the components to the boards, and the invention will be described in connection with this usage. However the invention may also be used in connection with other applications such as, for example, the curing of thick film materials; the curing of epoxies; the melting of preforms; or the cleaning of printed circuit boards with solvents.
Reflow soldering has become an important way of soldering leadless, surface mounted components to printed circuit boards. Reflow soldering refers to the process of remelting solder cream deposited on the circuit board in order to achieve the solder connection between the metal traces of the printed circuit board and the leadless components which have been placed on top of the solder cream. The heat required to remelt the solder cream can be applied in various ways. One of the preferred ways is by vapour phase treatment of the printed circuit boards. In vapour phase treatment, the heat is provided by the latent heat of condensation given off by a vapour as it condenses. The vapour is formed by boiling a stable, generally inert liquid such as one of the fluorinated liquids. Examples of such liquids are perfluorotrianylamine (known under the tradename "FC-70" and sold by 3M) or perfluoropolyether (known under the tradename "Galden" and sold by Montedison/Montefluos. The liquid is boiled in a sump at the bottom of a treatment tank. The vapour produced by boiling the liquid has a clearly defined temperature which must be higher than the melting temperature of the solder. The vapour collects in the tank over the boiling liquid and the printed circuit board is passed into the tank, through the vapour, and out of the tank by suitable conveyor means. The temperature difference between the board which is usually at room temperature, and the vapour, which is at the boiling point of the liquid, causes the vapour to condense on the board, giving up heat as it does so which melts the solder. The heating of the solder is uniform and quick, usually taking between fifteen and forty-five seconds. The soldering process employing the vapour is called vapour phase soldering.
The liquids employed to provide the vapours are generally inert so as not to react with the materials in and on the circuit boards. They have a high enough boiling point to melt most solders used in the electronics industry. They are generally safe to work with and are stable. They are also expensive.
Processes at present used in vapour phase soldering are costly because some of the liquid used to produce the vapour is always being lost and must be replaced. Some of the liquid is lost through accidental spills, and by being captured in filters used on the apparatus. Some liquid is also lost by being carried out of the apparatus by the conveyor and the workpieces such as circuit boards. Most of the vapour that condenses on the circuit boards, and on the conveyor, drains off within the apparatus but some residue liquid remains on the circuit boards and the conveyor and is carried out of the apparatus. It is known to upwardly incline the conveyor toward the exit of the treatment tank to facilitate draining of the liquid. However the problem of losing liquid by the residue liquid being carried out of the apparatus on the circuit boards and conveyor remains.
Liquid is also lost from known apparatus by evaporation. As the liquid boils, the molecules gain kinetic energy and may escape from the liquid. These molecules may then escape from the tank through entrance and exit openings in the tank. The loss of liquid in this manner can be considerable. To try to prevent this loss, apparatus has been designed to provide a substantial distance between the surface of the boiling liquid and the entrance and exit openings. However, such apparatus is quite large. Apparatus has also been designed to employ condensing means within or adjacent the entrance and exit openings to cool and condense the escaping molecules and return them to the liquid. The condensing means are however expensive. Apparatus has also been designed to employ a less costly secondary liquid to produce a lighter, secondary vapour that overlies the primary vapour, blanketing it to prevent loss by evaporation. The use of a secondary vapour reduces the loss of the primary vapour. However the secondary vapour loss is still substantial. In addition, because the primary liquid has a higher boiling point than the secondary liquid, the secondary liquid tends to decompose at the interface between the liquids producing chemically active products which attack the printed circuit boards and also producing toxic products.
Liquid is also lost from known apparatus through aerosol formation. Some of the vapour condenses onto dust particles or ions within the treatment tank. These particles are not affected by any condensation means employed by the apparatus and can be carried out by the apparatus through the exit opening by convection currents. The convection currents can be formed by temperature differences, and aided by mechanical movement of the circuit boards and conveyor.
Another problem associated with known processes in vapour phase soldering is the time required for the actual soldering within the apparatus. The longer the soldering time required, the thicker the formation of the intermetallic layer in the solder joint, and therefore the weaker the joint. It is therefore desirable to keep the intermetallic layer as thin as possible, particularly when forming small solder joints, by speeding up the soldering process. This can be accomplished by using liquids with higher boiling points. However such liquids are more expensive making the operation more costly.
A further problem with known processes is the formation of hot spots on the heating surface used to boil the liquid. The localized hot spots can be caused by using improperly sized heaters or more commonly, by flux residues falling off the circuit boards and onto the heater surface and carbonizing. The hot spots can cause overheating of the liquid causing it to decompose and break down into chemically active and toxic products. Specific heating means for boiling the liquid have been designed to try to avoid the hot spot problem. While successful, they add further to the cost of the machines.